Nikon D200: Long Exposure noise

[dang]

Mike wrote:

They requested an NEF file so I shot another test image (ISO 200, f3.5, 310 seconds). There appears to be less noise because the image is almost completely black, but after playing with levels you can see the noise is still there (not to mention plenty of magenta discoloration) if you click on the image below.

Sorry I should have thought about mentioning high ISO to begin with, but kept getting distracted for various reasons. The question is though, how does shooting
with a lower ISO work in real situations? You'll now increase exposure time to compensate, and the purple blobs will increase due to the sensor over heating. This pretty much rules out using it for astrophotography (or other fairly long exposure night photography), since Nikon states it's normal. It seems Canon sensors might still be slightly less prone to the problem, although far from perfect. Might be time to consider a film body for this type work?

[mikelong]

Mike wrote:

Might be time to consider a film body for this type work?

Thanks Tom, looks like I'll be dusting of my twenty-year old FM2 since it appears digital still has a ways to go in this department

[mostschaedel]

Hi Mike!
Astronomers who use DSLRs for long exposure images of the sky know your problem very well.
Its called Amplifier glow. Means, the electronics sourrounding the CCD-chip emit heat and Infrared radiaton.
Depending which camera you use there are more or less points glowing with more or less brightness.

Canon has a very good reputation in the amateur astronomers community because their DSLRs have very
minimum amp glow and very minimal thermal noise.
How do you solve this problem.
Making so called dark frames and subtracting them from the image with special software
like ImagePlus, Astroart, MaximDL, IRIS etc...
A darkframe is an image like you made already. It has the same exposure time as the light frame, should be made
with the lens cover on and should be made at the same temperature as the light frame!
Hope this helps, if you need more support give me an E-mail.
Gerald

[jestev]

That's from the sensor heating up. Make sure to keep the camera cool during long exposures and use the long exposure NR selection. It'll take a second exposure for the same amount of time and subtract it from the original shot. This is so the sensor will heat up again just like in the original exposure and then delete itself from the original. It takes twice as long, but it's worth it. Just don't move the camera from where you took the original exposure -- everything must be the same (temperature, humidity, etc.).

Hope that helps.

[dang]

Interesting discussion, and a good jog to the memory since I now recall reading something a few years back concerning this. If I recall, doesn't Canon (possibly others) make modified "cooled sensor" versions of some bodies? Might have to do some research in case this comes up again...

On a side note, I thought I'd pass on this gallery of star trails I enjoyed viewing: http://www.pbase.com/elroyie/star_trails

Good luck, and hope you get things sorted.

[doady]

I think this is the same reason why dSLRs with CCD sensors don't have Live View, a feature where the sensor is continuously gathering light. Camera's with the live view feature use CMOS (or "NMOS") sensors instead.